Method of oil recovery



lime 25, 1946 n K H. ANnRl-:sz-:N A 2,402,538

vMETHD 0F OIL RECOVERY Fuga sept. 14, 1940 NVENTR ,Kuri Hnruresen BY* APatented `)'une 25, 1946 Kurt H. Andresen, Bradford, Pa.. 355181101', by

mesne assignments. to Essex Royalty Corporation, New York, N. Y., acorporation of Dela- Application September 14,- 1940, Serial No. 356,736

Claims.

l This invention relates to a method of recovering oil from subterraneanoil-bearing sand strata of different permeabiiities'by injecting a fluidunder pressure into an input well tc force oil Vthrough the strata intoan output well from which oil is recovered, such oil producing being'commonly known as production by secondary recovery methods, and moreparticularly the invention relates to a method of increasing theeiiiciency of the producing operations by -decreasing the fluidconductivity of more permeable non-oll-producing strata adjacent to lesspermeable oil-producing strata.

ne method employed to produce oil from oilbearlng sands is to force afluid, such as water or a gas, into an input weil and through theoilbearing strata to force oil to flow through the strata to an outputor producing well from which oil or a mixture of oil and uld is taken.In accordance with this process a series of input wells approximatelyequidistant from a producing wel] are'drilled and exposed to the sameoil-bearing strata. Fluid is forced down the inputwells and out into theoil-bearing strata under sufficient pressure to drive the oil throughthe strata into the output or producing well.

In carrying out this method of oil. recovery serious diiiicultles havebeen encountered when a stratum or strata permit now of driving duid tothe output well without forcing a suilicient amount oi oil therefrominto the output well. Oil-bearing strata having different permeabilitiesand so different degrees of uid conductivity may lie closely adjacentone another and be exposed to the same input weil opening. Under thesecircumstances the driving fluid will drive the recoverable oil from themore permeable strata before the recoverable oil is driven from adjacentless permeable strata leaving substantial quantitles of recoverable oilin the less permeable strata. As more permeable strata are emptied ofrecoverable oli they become more permeable to the less viscous fluid ofthe fluid drive and act to short-circuit the driving fluid operation.Thus, it becomes necessary to pump in more and more pressure fluid foreach barrel of oil produced at the output well and a condition may bereached where the cost of pumping and handling the quantity of drivingfluid necessary to produce a barrel oi oil is greater than the value ofthe oil thus recovered.

Also` since the rate of oil production under fluid drive is a functionof the conductivity of the sands and the pressure drop across them, itrequires a higher pressure to maintain desired rate 4 of production fromthe tighter or less permeable olproducing sands. But if after the morepermeable strata are depleted the pressure of the driving fluid isincreased to increase the rate of production from the tighter sands theilow oi driving fluid through the loose sand becomes so great as torender the production unprofltable.

The process of the present invention provides in general that when theratio of driving fluid to oil produced at the output well becomes undulyhigh due to depletion of more permeable strata. adjacent less'permeablerecoverable oil-bearing strata, the depleted more permeable strata areso selectively sealed or their conductivity is so reduced that wheninjection of driving fluid is resumed a greater proportion of thedriving uid will now through the non-depleted oil-producing strata.Further, the seal provided is such that the pressure under which thepressure fluid is injected into the wells may be appreclably increasedwithout increasing the driving fluid oil ratio. Thus, the rate of oiiproduction from remaining tighter (less permeable) oil-producing sandsis increased. l

The problems involved in selectively sealing sand strata locatedhundreds of feet below the surface of the ground are diiilcult. Methodsof selective sealing utilizing the relative positions of the adjacentdepleted and non-depleted strata and their respective depths below theground surface are relatively unsatisfactory where the wells are deepand possibilities of error in the depth measurements are increased. Yetthe selectivity of the sealing is necessary to prevent sealing ofproducing strata. Also, to oiIset the necessity for successivetreatments the dam formed by the sealing process must be permanent andable to withstand high pressures when the uid drive is carried out atsuch high pressures.

Another characteristic of the well that makes selective sealing of thestrata diilicult is that the walls of the well exposed to the producingstrata are "shot" by setting oli a high explosive which ruptures,breaks. and produces fissures in the sand extending undetermlnabledistances in from the well hole proper and also increasing the effectivediameter of parts of the well hole undeterminable amounts.

In the present embodiment o! the invention these problems are solved byinjecting into the input weil a solution of a precipitatable materialwhich is caused to flow preferentially into the pores of the morepermeable strata and by a subsequent and controlled chemical action thepreclpitatable material is precipitated from the solution in the poresto decrease the conductivity of the strata into which the solution isiiowed. Ihe precipitation is accomplished by injecting into the pores ofthe solution-containing depleted stratum under controlled conditions achemically active material capable of yielding a reagent or ion whichreacts to precipitate the precipitatable material from the solution inthe pores of the depleted stratum. The chemically active material andprecipitatable material and 'their concentrations are so chosen that theprecipitate is formed at a relatively slow rate.

More speciilcalhf, an alkaline sodium silicate solution is first flowedinto the input well in such manner that it selectively enters the poresof the depleted more permeable strata and the alkalinity of the solutionis then gradually reduced to cause a water insoluble gelatinousprecipitate containing SiO: to form slowly in the pores. In the presentembodiment the alkalinity of the so` lution is reduced by a reagent thatis in effect a weak acid and so only` slowly yields hydrogen ions tobring about the precipitation, Carbonio acid is satisfactory and theprocess is carried out by flowing carbon dioxide gas into the weil insuch manner that the gas selectively enters the v' pores which havealready been penetrated by the solution. The carbon dioxide dissolves inthe solution to form carbonio acid and to reduce the alkalinity of thesolution. The dissolving action is increased by the alkalinity of thesolution, by the pressure under which it is introduced and by the verylarge exposed surface of the solution in the pores exposed to the wellhole,

Sometime after the carbon dioxide has dissolved the precipitate startsforming in the form oi' a gel having a viscosity far greater than thatof waterso great that even when subjected to a high pressure drop acrossthe gel in the pores it does not move or migrate through the poresbecause of its high viscosity. The eil'ect of placing in the pores a gelwhich, although having a small tensile strength, has a high viscosity.ls shown by the equation of Darcys law, which is as follows:

KA (P, P2) Q uL where Q=iow in unit volume per unit of time =Darcysconstant (permeability) A=unit area Pi-Pz=pressure drop along unitdistance u=coeiiclent of viscosity L=1ength Whereas the relativeviscosity of water in centipolses may be considered as unity. theviscosity of the gel in centipoises would be in the order of millions.Thus, since the flow through the sand varies directly with the pressuredrop across it and inversely with the coeillcient of viscosity, bymaking the coeilicient of viscosity very large the tlow through may bemade negligible. Thus, the gel acts as a dam and is as effective asthough it were rigid.

With regard to the question of the concentration of the alkaline sodiumsilicate solution that is introduced, the following factors, amongothers, require consideration. The more concentrated the solution themore viscous the gel subsequently formed from it, and therefore aconcentration is picked suiliciently high to obtain a suiiicientlyviscous gel. But the concentration of the solution is kept low enoughthat the viscosity of the solution as it is introduced is not so high asto require too high a pressure to force it luto the pores of the morepermeable strata. In this connection, however, it is noted that theviscosity of the solution being higher than that of water has a greaterselectivity to enter the more permeable pores of the strata under agiven pressure than would a less viscous liquid because the higher vtheviscosity of a liquid the more sensitive it is tov differences inpermeability.

But since it is desirable to have the gel form in the pores to somedepth from the exposed surface of the sand the concentration of thesolution is also kept suillciently low that the formation of the gel bythe carbon dioxide treatment is retarded, If the concentration isincreased beyond a certain point the gel may so form near or at thesurface at which the carbon dioxide is introduced as to preventpenetration of the carbon dioxide into the solution in the pores furtherremoved trom the exposed surface of the sand.

For the same reason the medium or chemical used to bring Yabout thegelling action is selected to cause the gelllng to take place slowly.Carbon dioxide has such a favorable action because in dissolving in thesolution the acid radical that it yields can develop only to a limitedconcentration. Thus, since the carbon dioxide gas is readily soluble lnthe solution, and since the acid that it yields is relatively weak, itpermits the gelling reaction to take place at the desired slow rate.

One solution which gives satisfactory results has approximately thefollowing composition:

Per cent NazO .39 SiO: 1.25 H2O 98.36

Figure 2 shows an arrangement of apparatus for supplying the chemicallyactivegas to the oil-bearing strata.

Referring to the drawing, and particularly to Figure l, the numeral iindicates a Water input Well whichextends from the surface 5 of theground to oil-bearing strata 2, 3 and l. As suggested above, a number ofthese input wells are usually drilled around a producing or output wellat spaced intervals approximately equidistant from the output well. Eachof the input wells l is provided with a. pipe line 6 extending from thesurface of the ground through a packer l to the oil-bearing strata.Driving iiuid such as water is pumped into the well and is caused to nowout only into the strata 2, 3 and l by the presence of the packer lwhich may be of any desired type and which prevents the water frompassing ul wardly through the input well.

The water from a suitable source (not shown) is injected into the wellthrough a branch line 8 and flows through the pipe E into the strata 2.3 and 4, forcing oil therein towards the producing well from which theentering oil and water is taken either by pumping or by flow under theimpetus of the fluid drive. The rate of flow ol water is measured by awater meter 9 and isregulated by valve Il.

Stratum 1 is represented in the drawing as being the more permeable. andhence as the water flooding operation proceeds this stratum will bedepleted of recevable oil beiore the strata I and l are depleted. Sincethe viscosity of water is less than that of oil, the rate of iiow ofwater through the stratum 2 increases as the stratum is depleted ofrecoverable loil and the ratio ot driving fluid to oil produced mayincrease to an imeconomical point. e

.In accordance withv one embodiment oi the method of the presentinventionl when stratum 2 is thus depleted, as indicated by asubstantial increase in the wateroil ratio of the iluid produced at theproducing well, the well is treated in the-following manner. The supplyo! water is shut ofi. A tank I3 containing a measured quantity of sodiumsilicate solution is connected to line 'l and the solution is caused todow into the well. Tank I l may be provided as shown with a vent valveIl, pressure gage I5, air supply line it, and air control valve i1.

The solution normally ilows under hydrostatic pressure down through thepipe 6 and the hydrostatic pressure may be lowerl than the pressure oi'the water drive under which the well had been operating. Thus, thesolution forced down has a selective tendency to enter only the moreApermeable strata under Vthe reduced 4pressure at which it isintroduced. As above pointed out, this selectivity is further enhancedvby the fact that the viscosity of the solution is greater than that ofwater.

Whereas in most cases thehydrostatic pressure of the fluid providessuilicient head to force the solution into the more permeable strata,w'hensuch hydrostatic pressure is not suillcient it may be aided bysupplying air under pressure to the tank through the line i8 andcontrolling such air flow to obtain the desired rate oi' flow of thesolution into the well to maintain the selectivity as above described.

I'he quantity of solution used is sumcient to fill the pores ci thestratum to a desired distance from the well hole to obtain the desiredpiugging. One of the factors influencing the quantity of solution usedis the thickness of the Astrata to be sealed. The thickness of thestrata may have been at least roughly determined by core analyses madeduring the drilling of the well or of a well in the area of the wellbeing treated.

After the solution has been introduced the pipe t is cleared of solutionby forcing air therethrough and the level of solution is forced down toJust below the more permeable strata. which in the present disclosure isstratum 2. Pipe 6 is disconnected from branch line l at. for example,union IB` As shown in Figure 2, a pipe i9 of smaller diameter than thepipe i and provided with a packer 2li is inserted within the pipe 6.Carbon dioxide gas is then forced through the pipe i9 into the stratum 2to precipitate the precipitatable material of the solution. Theadvantages of using the inner, smaller pipe are twofold. since thevolume of the inner pipe is small there will be relatively littledilution by air of the chemically active gas as it passes through thepipe. Furthermore, the inner pipe prevents the gas from coming incontact with residual solution which adheres to the inner surface of thepipe 5 and so avoids the formation of quantitles of precipitate in thepipe 8 which might be later tlushed into the well to clog the pores ofthe faces of the strata l and l when water injection is resumed.

vout driving the solution ahead of it.

It has beenfound desirable to supply the gas to the input well at arelatively low rate. In one case where satisfactory results wereobtained. gas was supplied at the rate of cubic feet per hour (based onatmospheric pressure) over a period of ten hours. Such a slow rate notonly makes the gas flow more selective but also permits it to seepthrough and dissolve in th'e solution in the pores and to penetrate thepores with- Using such a dilute solution of sodium silicate forms a gelthat has a Suiliciently high viscosity so that the dam formed issubstantially permanent, and has an advantage that the time for gellngvaries with the variation in the alkalinity (the pH) so long as the pHis above or below a narrow range. Thus, as the pH is reduced by theabsorption of the CO: gas, the setting time for the gelling iscorrespondingly reduced. In the narrow range the change of pH causespractically no change in the setting time of the gel. Below this narrowrange, as the pH is further reduced. the setting time correspondinglydecreases.

With higher concentrations of sodium silicate the range above-mentionedis wider and in this range variation of the pH does not vary the settingtime of the gel. Above this wide range there is a narrow range of pH inwhich a relatively small change in pH causes a relatively large changein setting time, and above this latter range variation in the pH has noeilect upon the setting time.

Another advantage of using such a dilute sodium silicate solution isthat as the pH of the solution in the pores is reduced to bring aboutthe gelling action relatively little change in the viscosity of thesolution takes place during the early stages of the gelling action,after which the gellng action proceeds at a relativelyrapid rate. Inmore concentrated solutions, however. as the pH is reduced to bringabout gelling action the viscosity of the solution starts increasingalmost simultaneously with the reduction of the pH. In other words, withthe concentrated solutions the viscosity is more nearly a linearfunction of time. Thus. by using such a dilute solution oi' sodiumsilicate and avoiding Increase in viscosity of the solution as the CO1gas is flowed into the solution in the pores. the penetration,dissolving and ditluslon of the C01 throughout the solution in the poresis improved.

The solution treatment leaves a liquid body 2i of the solution in thepit of the well. To prevent the incoming gas i'rom forming a precipitatewith the liquid body 2i, which precipitate might later be deposited onthe faces of the non-depleted strata 3 and 4 or even ll the well holewith the precipitate and prevent ilow of flooding water thereinto. astrong alkali may be injected into the liquid body 2i to render thesolution so alkaline that no precipitate will remain in it. The alkalinetreatment may' be carried out by dropping sticks of sodium hydroxideinto the pit containing the liquid body 2i.

After the gas treatment and caustic treatment, a period of time isallowed to permit the precipitation to complete itself and theprecipitate to set before the fluid drive is resumed. A period of sixtyhours was permitted to elapse in the present example alter gas treatmentin order to make sure that the reaction was complete before resumingwater injectionY The optimum rate of gas supply and the time requiredfor completing the reaction will, of course. vary with the type ofreagent used, the concentration of the solution used. and the type offormation to which the solution is supplied.

I'he use of a gaseous precipitating medium having inherently a muchlower viscosity than a liquid medium insures penetration of theprecipitating agent into the pores so that the precipitation is causedto take place throughout the solution.

As above noted the concentration of the treating solution is preferablysuch that the viscosity of the solution is kept relatively low so thatit readily enters the permeable strata. under the relatively lowpressure used. Also it is kept sudlciently low that the reaction ratebetween the solution and the'subsequently dissolved gas isrso slowthatit does not interfere with the absorption of the gas and themigration of the dissolved gas to the solution farthest from the wellhole. This permits the subsequent precipitation to take place throughoutthe solution in the pores. Yet the concentration is kept sutilcientlyhigh to form within the pores a gel that has a high enough viscosity toprovide a permanent seal, i. e. one which will not break under a highpressure fluid drive if such is used.

Other materials than sodium silicate that may be used in carrying outthe method of the present invention are, for example, solutions ofaluminates and titanium salts. The kind of gas used will depend upon thenature of the solute used in the impregnating solution. If the soluteused depends for its solubility upon the alkalinity of the solution, assodium silicate does, then an acid forming gas may be used. such ascarbon dioxide illustratively mentioned above, or one of the oxides ofsulphur, or the hydrogen halides, H28. or the oxides of nitrogen. Wherethe maintenance of the iprecipitatable material in solution depends uponfactors other than alkalinity, other appropriate precipitant gases wouldbe used.

Referring now to another embodiment of the method of the invention bywhich the use of the inner pipe i9 for theintroductlon of the gas iseliminated, the solution is introduced in several stages and the gastreatment is likewise made in several stages. Thus, after a part of thesolution has been introduced through p/lpe line 6, it is immediatelyfollowed by a CO2 treatment, which forces solution remaining in line 6into the pores nd then itself enters the solution in the pores.

en another portion of the solution is flowed In. `forcing ahead of itthe solution previously introduced and gas treated. Then follows asecond gas treatment. These treatments are repeated as desired andfollow one another at sutliciently short intervals that the gel does notform until after the last treatment. After the last gas treatment, thesolution with the dissolved gas is allowed to stand as above describeduntil the seal is complete.

The following example is given of the manner in which the aboveprocedure may be carried out on a water intake well where twenty-threebarrels of the solution were estimated as. the correct quantity: Thesupply of flooding water to the intake well is turned olf. The tank i3is then connected up to the line 8 and fourteen barrels of the solutionare allowed to iiow in under hydrostatic pressure. This first batch ofsolution sweeps the flooding water from the line 6 and flushes it out ofthe pores of the oil depleted permeable strata and nils the pores aroundthe well hole full of undiluted solution. With this first step that partof the solution diluted by the 8 flooding water in the well at the startof the treatment may be ineffective but the amount of solution used forthe rst step is large enough to leave undiluted solution in the well andpores immediately adjacent the well hole.

Two hundred pounds of CO2 may now be introduced at a slow rate, forcingthe solution left in the line E into the pores and subsequently itselfdissolving in the solution in the pores. The remaining nine of thetwenty-three barrels of solution are now introduced in two treatments of4.5 barrels each, and each followed by 200 pounds of CO2 as described.After the last CO2 treatment, sodium hydroxide sticks are dropped intothe well hole to render the solution in the well hole so alkaline as toprevent any subsequent precipitation in the well hole during the timeallowed for the prf :ipitation to take place in the pores.

An advantage of this embodiment is that the solution and gas are mixedin sections and the gas does not have to diffuse through the s0- lutionto reach the solution farthest from the well hole.

The method of the invention may also be used to selectively seal themore permeable non-oilproducing strata of sands from which oil has beenproduced by means of a gas (air) drive. But in instances where the morepermeable strata of gas intake wells are sealed the procedure ismodified so that when the gas drive is resumed there is no chance forany of the solution to enter the remaining oil-producing strata. Thisprecaution is taken because water in the presence of the gas drive actsto seal or plug the sand.

l'n practising the invention on a gas intake well, therefore, inaccordance with one embodiment of the invention, after the gas input hasbeen shut off and before the sodium silicate solution is injected, anorganic liquid is introduced into the well approximately to the level ofthe loose or oil depleted sand. This liquid preferably has a specificgravity higher than that of the aqueous silicate solution, is immisciblewith water and has a vapor pressure suiliciently high at welltemperature so that evaporation can take place. One liquid satisfactoryfor performing this function is carbon tetrachloride or a mixture ofcarbon tetrachloride and gasoline.

After this liquid is injected 4into the Well the sodium silicate-carbondioxide treatment is carried out as above described. After the reactionperiod has passed and the seal formation ls com- Dleted the excessliquid in the well is driven ofi. This excess consists predominantly ofthe protecting liquid initially introduced.

The protecting liquid having a specific gravity heavier than that oi'the solution serves to prevent the sodium silicate solution frompenetrating the well below the level of the loose sand which is intendedto be plugged and the immisclbllity of the solution with the protectingliquid also serves this purpose.

The use of a protecting liquid having a high vapor pressure permitsevaporation of any liquid remaining after the well is pumped so thatafter the Kas repressurlng is resumed, any liquid remaining would causeno permanent plugging as, lor example, water might.

When injection of water. to the inlet well is resumed, the water-oilratio at the producing well is substantially reduced and oil productionby flooding is again economically feasible. Also the pressure underwhich the fluid drive is opersteps of flowing 'solution in place stepso! flowing into ated may be increased to increase the production rate ifthe remaining strata are much tighter than the plugged strata. -Thus themethod o! the present invention makes available oil present in the lesspermeable strata which would not otherwise be practically available.High selectivity and a deep penetration ot the precipitate is obtainedby using a gaseous precipitant and a relatively low impregnationpressure.

i. The method of selectively sealing more permeable undesirablefluid-passing porous sand strata exposed to an input well o! an articialiiuid drive oil producing system without sealing desirable fluid-passingless permeable strata exposed to the well, comprising the steps o!stopping the fluid drive into said input well. iniectq ing into saidinput well a dilute alkaline solution of sodium silicate under suchreduced pressure that the solution preferentially flows into the poresof the more permeable undesirable iiuidpassing strata and not into thepores of the less permeable desirable fiui strata. subsequentlyinjecting a gas into said input well at such rate and pressure that itselectively enters the solution-filled pores and dissolves in thesolution to reduce the alkalinity o! the solution and slowly cause thesolution to gel in lowing time for the precipitating place. and makingthe concentration ot the solution such that it contains approximately1.25% SiO: by weight.

2. The method of selectively sealing oi! the now oi uuid throughundesirable fluid-passing porous rigid sand formationsexposed to an oilwell without plussinsr desirable porous oil-producing strata exposed tothe well. comprising the into the well a relatively dilute solution oisodium silicate (approximately 1.25% SiO: by weight) under conditions ofsuch slow controlled iiow that the solution enters the more permeablestrata to be plugged to the exclusion lthe less permeable strata, owingcarbon dide gas into said well under such slow now that it selectivelyenters the pores o! the more permeable strata illled with the solutionto reduce the pH thereof, and maintaining said treated in said poresuntil the solution iorms in the pores a uld silicic acid gel o!suiiicient viscosity and depth to act as a permanent barrier to the ow.o! duid through said pores.

3. The method ol selectively sealing oi! the how of nuid throughundesirable duid-passing porous rigid sand formations exposed to an oilwell without plugging desirable porous oil-pro-r ducing strata exposedto the well. comprising the the well a relatively dilute solution ofsodium silicate (approximately 1.25% BiO: by weight) under conditions otsuch slow controlled new that the solutionententhemoro wr permeablestrata to be plugged to the exclusion o! the less permeable strata,iiowing carbon di. oxide gas into said well under such slowr now that itselectively enters the pores o! the more per` meable strata nlled withthe solution to reduce the pH thereof. repeating said solution andgastreating operations successively until the desired amount of solutionhas been iiowed into the pores. and maintaining said treated solution inplace in said pores until the solution forms in the pores a iluidsilicio acid gel of sumcient viscosity and depth to act as a permanentbarrier to the flow of duid through said pores.

4. The method of selectively sealing oi! the flow of fluid throughundesirable iluid-passing porous rigid sand formations exposed to an oilwell without plugging desirable porous oil-producing strata exposed tothe weil, comprising the steps of flowing into the well a solution ofsodium silicate of such dilute concentration that variation oi the pH ofthe solution through practically the entire range causes a variation inthe setting time of the silicio acid gel under conditions of such slowcontrolled ilow that the solution enters the more permeable strata to beplugged to the exclusion of the less permeable strata. iiowing carbondioxide gas into said well under such slow flow that it selectivelyenters the pores oi the more permeable strata lled with the Vsolution toreduce the pH thereof. and maintaining said treated solution in place insaid pores until the solution forms in the pores a iiuid silicio acidgel oi sumcient viscosity and depth to act as a permanent barrier to thenow of fluid through said pores.

5. The method of selectively sealing oil the flow of uuid throughundesirable fluid-passing porous rigid sand formations exposed to an oilwell without plug desirable porous oil-pro-n ducing strata expos to thewell. comprising the steps o! llowing into the well a solution oi sodiumsilicate of such dilute concentration that variation of the pH of thesolution through practicaily the entire range causes a variation in thesetting timeoi the silicio acid gel under conditions oi such slowcontrolled iiow that the solution enters the more permeable strata to bePlugged to the exclusion o! the less permeable strata, ilowing carbondioxide gas into said well under such slow ilow that it selectivelyenters the strata iilled with Y repeatins saidporco.

KURIER..`

